Plasma etching of thin film layers on a workplace such as a semiconductor wafer can be performed in a plasma reactor having an inductively coupled plasma (TCP) source. Such a reactor may be referred to as an ICP reactor. An advantage of an ICP reactor is its ability to produce very high plasma density for a high etch rate and superior productivity. An ICP reactor typically has an external coil antenna that couples RF plasma source power into the chamber interior through a dielectric window. The materials used in advanced memory technologies for magnetic random access memories (MRAMs) have desirable magnetic properties and include (but are not limited to) Co, Pi, Pd and Ru. Such materials may be referred to as advanced memory materials. These materials tend to accumulate on the dielectric window as a metal film, because they resist combining with etch process gases (or plasma by-products formed from the etch process gas) to form gaseous or volatile species that would be readily removed by evacuation or pumping. Therefore, in this specification, such advanced memory materials are referred to as being “non-volatilizable”. Ordinary metals not included in the group of advanced memory materials generally react with a plasma to form either: (a) gaseous or volatile species that are readily removed or (b) compounds that accumulate on the dielectric window as non-metallic materials that do not significantly affect RF coupling through the dielectric window. Unlike ordinary metals, the advanced memory materials (the non-volatilizable metals) do not in general react with a plasma to form compounds, and instead accumulate on the dielectric window as a metal. In many cases, the non-volatilizable metal or material is located in a lower thin film layer, while the overlying thin film layers are ordinary materials not included in the group of advanced memory materials, that are readily removed during an etch process. Such ordinary materials may include non-metallic materials (such as SiN, TiN, polysilicon, or silicon dioxide, for example), or ordinary metals (such as Ta, Ti, or W, as a few examples) which are not advanced memory materials.
The non-volatilizable metal layer is exposed upon removal (etching) of the overlying thin film layers. When the lower layer containing the non-volatilizable metal is uncovered and exposed to the plasma, the non-volatilizable metal is released into the plasma chamber and deposited on interior chamber surfaces, including the dielectric window of the ICP reactor. The non-volatilizable metallic material builds up over time on the dielectric window. The coupling of RF plasma source power through the dielectric window is altered or attenuated by the buildup of metal on the window. The metallic accumulation on the dielectric window can shift RF coupling and plasma density in an uncontrollable manner, therefore causing process drift.